I am trying to print to PDF at a scale of 1:300 on 18’‘x24’’ paper. Unfortunately the options that come up for printed scale are 1:100, 1:200, 1:500 etc. so it skips 1:300… I clicked on the custom option but it doesnt seem to do anything.
Normally when using layouts in Rhino, you want to print at 1:1, it is the detail that you want to scale to any scale you wish. So in your case, set the layout size to 18"x24" (Arch C), create your detail, select the detail itself, and in the properties window you can set its scale to 1:300 (and lock that scale if you wish so you don’t mess it up).